The LAS measurements of the structural parameter of the refractive index of air (Cn2) were used to calculate the sensible heat flux. The latent energy flux is the difference between Rn-Fs and Fh and an energy flux of 12.2 MJ m-2 translates into approximately 5 mm of evaporation..64 Fig 6.17.
Introduction
The trees could survive long periods without water and total evaporation was also found to exceed rainfall. It enables agronomists and hydrologists to estimate total evaporation from standard weather station data commonly collected on a nationwide scale.
Energy Balance Method of Determining Total Evaporation
- Energy and Evaporation
- Net Irradiance
- Soil Heat Flux
- Sensible Heat Flux
- Latent Energy Flux
- Energy Balance Closure
Materials in the atmosphere and Earth's surface absorb some solar radiation and will emit radiation themselves. Clouds and various gases form some sources of infrared radiation in the atmosphere.
The American Society of Civil Engineers (ASCE) Automatic Weather Station
- Introduction
- Terminology Relating to Evaporation
- The Penman Equation
- The Penman-Monteith Equation
- The FAO-56 Penman-Monteith Equation
- The ASCE-EWRI Penman-Monteith Equation
The FAO-56 Penman-Monteith equation is a representation of the physical factors at a height of 2 m and physiological factors at the surface control. These continued efforts and developments have contributed to the ASCE-EWRI Penman-Monteith equation increasingly becoming an industry standard.
A Review of Large Aperture Scintillometry
- Introduction
- Principles of Scintillometry
- Structure of the Atmosphere and the Application of the Monin-Obukhov
- Heterogeneous Surfaces and the boundaries of the Monin-Obukhov
- Determination of Effective Height (Z LAS )
- Roughness Length and Zero Plane Displacement
- Roughness Length
- Zero Plane Displacement
- Footprint Concept Applied to the LAS
- Saturation
- Advantages and Disadvantages of Scintillometry
The structure of the atmosphere and the application of the Monin-Obukhov Similarity Theory (MOST) Obukhov Similarity Theory (MOST). Heterogeneous surfaces and the limits of Monin-Obukhov Similarity Theory (MEST) Obukhov Similarity Theory (MOST). The roughness length is an expression that describes the aerodynamic roughness of the earth's surface.
It depends on the height of the roughness elements as well as the distribution. In practice, d is a parameter that can be difficult to derive and is one of the biggest potential problem areas (except for short vegetation or for cases where the measurement height greatly exceeds d) when applying MOST ( Foken, 2006). In practice, d is a parameter that can be difficult to derive and is one of the biggest potential problem areas (except for short vegetation or for cases where the measurement height greatly exceeds d) when applying MOST ( Foken, 2006).
When this occurs, the relationship between scintillations and the structure parameter of the air refractive index fails (Kohsiek et al., 2005).
Materials and Methods
- Description and location of the Two Streams area
- Site Descriptions
- ASCE-EWRI Automatic Weather Station (AWS)
- Scintillometer Transect
- Energy Balance Weather Station
- Equipment and Data Recording
- ASCE-EWRI AWS Measurement System
- The LAS Measurement System
- Energy Balance Weather Station
- Tree Growth
- Reliability of the Instrumentation
- Data Analysis
It is critical that the wind speed measurements are clear of the tree canopy and as close to the height of the LAS beam as is practically achievable. This is critical, especially for the transmitter as any movement has an exaggerated effect on the beam spread over the length of the path. A limitation of LAS is that it does not provide direction of sensible heat flux.
Net radiation, ground heat flux, and horizontal wind speed over the canopy are additional measurements required to calculate the latent energy flux using Winlas software and the abbreviated energy balance (Eq. 2.1). Wind speed is critical in calculating sensible heat from LAS, and a method to adjust for wind speed was sought. Ground flux measurements were installed at the base of the tower on the north side to avoid shadows from the tower.
Outlier data are easily identified as sensible heat values are usually above net radiation for the first 10-minute readings in the morning.
Results and Discussion
Plant Growth
Energy Balance Weather Station
Temporal change in the relationship between soil heat flux (Fs) and net irradiance (Rn) showing the effect of weeds and canopy closure. Ground heat flux and net radiation are both negative at night, while the Earth's surface emits infrared radiation. The Earth heat flux indicates that the initial cooling of the Earth's surface as the sun sets occurs rapidly and stabilizes towards the early morning.
In 2006, the response of the soil heat flow in the morning and evening to changes in available energy is fast, but in 2007 it is a gradual response due to shading of the soil surface by vegetation. Wind speed is an auxiliary measurement needed to calculate sensible heat from LAS data. The wind speed at the site was not particularly high, although wind speeds above 10 m s-1 were recorded in shorter periods of time (Figure 6.7).
The resolution of the wind speed data is every 10 minutes to coincide with the LAS data measurements.
ASCE-EWRI Automatic Weather Station
This shows that where hourly data is available it should be used rather than the daily data and where calculations are carried out with only daily data a possible error (in the case of Two Streams) of 17 % in the data is possible. The ASCE-EWRI method of calculating ETsz gives results for daytime and nighttime (usually very small) conditions separately. The handbook for the ASCE-EWRI equations (Allen et al., 2000) recommends that the nighttime values be included in the calculation of daily ETsz from hourly values.
The daily values of ASCE-EWRI short grass reference evapotranspiration (ETsz) calculated using the daily formula vs hourly values summed to a day. The relationship between the daily values of ASCE-EWRI short grass reference evaporation (ETsz) and hourly values of ETsz summed up to a day but including night time values. The impact of including negative hourly nighttime values of ASCE-EWRI short grass reference evapotranspiration (ETsz) in a daily summary of ETsz values.
The impact of including the ASCE-EWRI nighttime short grass reference evapotranspiration (ETsz) values over the 7-month period from March to October is 15 mm with an average daily error of 5.7.
LAS Estimates of Sensible Heat
This is intuitively what would be expected since by the law of conservation of energy, the available energy must equal the sum of the sensible heat flux and the latent energy. A relative allocation plot showing the total daily distribution of available energy, sensible heat flux and latent energy in Two Streams from September 23, 2007 to October 28, 2007. Sensible heat flux and latent energy flux are assumed to be zero when the net radiation is negative.
This indicates that the latent energy flux was the dominant flux at the site during the wetter months. Temporal change in total daily latent energy flux using the LAS-based method from 22 August 2006 to 29 September 2007. Temporal change in total daily latent energy flux using the Priestley and Taylor (1972) method from 22 August 2006 to on September 29, 2007.
Relationship between the LAS and the Priestley and Taylor estimates of daily total latent energy from 22 August 2006 to 29 September 2007.
A Comparison between the Automatic Weather Station Reference
With the increase in tree height and leaf area, the crop factor was expected to increase, but the trend of the data shows a decrease in winter. This may be a seasonal influence and the crop factor may show an increasing trend as summer approaches. Many of the outliers below the regression line are the result of days when rain or wind affected the measurements.
The variability of crop factor data is not as great as it may appear from the graph. In both of these cases, the absolute differences in evaporation are small, but the ratio between the values is large, resulting in variation in the crop factor results. Ratio of daily AWS shortgrass reference evapotranspiration (ETsz) to total LAS evapotranspiration (ETLAS).
Total Evaporation, Reference Evaporation (ET sz ) and Rainfall
The Priestley and Taylor method, although relatively crude compared to the LAS measurements, verifies that the LAS measurements are within the range of what would be expected on site and gives confidence in the results obtained so far. The low rainfall in January and February indicates the rainfall variability at the site and the importance of using long-term measurements to evaluate evaporation and precipitation. The low precipitation in January and February will affect the comparison of evaporation and precipitation, but over time, as the individual values of accumulated ET and precipitation become larger, the impact of unusual weather conditions will be reduced.
Conclusion and Recommendations for Future Research
The LAS ET was compared with the grass reference evapotranspiration (ETsz) calculated from an automated weather station using the daily and sub-daily ASCE-EWRI methods. Outliers are mainly due to wind advection affecting the LAS sensible heat flux measurements or rain. When available, the hourly values of ETsz should be collected and used instead of the daily calculated reference evaporation.
Although less accurate, the Priestley and Taylor method provides a good validation of the LAS ET results. Total rainfall for the measurement period was 750 mm and ET estimated by LAS was 1250 mm. Results from LAS measurements confirm that total evaporation can be higher than total precipitation, and for this measurement period, total evaporation exceeded precipitation by 67%.
The soil's water-holding capacity also plays an important, long-term role in enabling the trees to survive.
Analysis of the short-term closure of the surface energy balance on short vegetation, Agricultural and Forestry Meteorology, In Press. Effective management of riparian zone vegetation to significantly reduce the cost of watershed management and enable greater productivity of land resources, WRC Report No. Regional calibration of the Hargreaves equation for estimating reference ET in a semi-arid environment, Agricultural Water Management.
Saturation Theory of Optical Scintillation by Strong Turbulence for Arbitrary Refractive Index Spectra, Journal of Optical Society of America. Sensible heat flux of irrigated areas in western Turkey determined with a large-aperture scintillometer, Journal of Hydrology. Using the Bowen ratio energy balance method for determining total evaporation over a grassy surface.
Behavior of the refractive index structure parameters near the ground, Journal of Optical Society of America.
